A premature atrial contraction (PAC) is a native depolarization arising in the atrium that is premature with respect to the basic sinus cycle. Also called atrial premature beats or atrial premature contractions, premature atrial contractions occur when a site in the atria other than the sinus node develops automaticity of a rate greater than that of the sinus node. This site from which the contraction originates is called an ectopic focus. An ectopic focus can usurp control of the atria from the sinus node for one or a few consecutive beats causing a single PAC or a “salvo” of consecutive PACs. If the ectopic focus remains in control for a long period, the heart condition is referred to as a focal atrial tachycardia.
PACs can differ from sinus-initiated atrial contractions in many respects, including in their timing, in the duration of their action potential and effective refractory period, and in their speed and direction of propagation through the atria. The action potential duration and effective refractory period of atrial myocytes shortens as the rate at which they are activated increases. Therefore, if a PAC occurs very soon after a previous atrial contraction, effective refractory periods around the atria can be significantly shorter for the PAC than they were for the previous contraction. The relationship between the effective refractory period and the previous interval duration may not be uniform throughout the atria. Therefore a PAC can cause dispersion of refractory periods. This means portions of the atria become excitable before other portions, maybe while a wave of excitation is still present. This condition sets the stage for reentrant arrhythmia.
PACs are often associated with the onset of atrial flutter and atrial fibrillation. A single PAC or a salvo of PACs can trigger atrial fibrillation or other reentrant atrial tachycardias. Additionally, the frequency at which PACs occur can be an indicator of susceptibility of the atria to atrial fibrillation or atrial tachycardias. PACs may also trigger arrhythmias by occurring at a moment when the atrium in not uniformly recovered from the previous beat.
Typically pacemakers do not recognize PACs as such. A PAC will either occur during an atrial refractory period and be ignored completely, or it will occur during an atrial alert period and be treated as a sinus P-wave. A pacemaker in DDD mode may track a PAC and thereby cause an irregularity in the ventricular rhythm. A pacemaker in DDI mode may be inhibited by a PAC, disrupting AV synchrony and setting the stage for a retrograde P-wave. There is a need for improved techniques for accurately detecting PACs and where appropriate, administering responsive pacing therapy.